1. Field of the Invention
The present invention relates to an image formation apparatus using an electrophotographic process, such as an electrostatic copier and a laser printer, more particularly, an image formation apparatus using an electrophotogrphic process of which a light-writing resolution is equal to or more than 1200 dpi. Also the present invention relates to an image formation apparatus in which light writing is performed based on image data obtained by applying halftone processing at a line frequency of equal to or more than 200 lpi to an input image.
2. Description of the Related Art
Conventionally, an image formation apparatus is disclosed in Japanese Laid-Open Patent Application No. 8-272197, which includes an electrophotographic photoconductor having a photosensitive layer on a support, charging means for charging the photoconductor, light-exposure means for irradiating light on the charged electrophotographic photoconductor, developing means, and transcribing means, wherein the product of the spot area of light radiated from the light-exposure means and the thickness of the photosensitive layer is equal to or less than 20,000 μm3.
Thus, an image formation apparatus and a process cartridge are provided which could obtain an image with high resolution and good tone.
The prior art characterized by satisfying the certain condition:Vc/Vo≦0.92 log(S)−0.018L−0.29is also disclosed in Japanese Laid-Open Patent Application No. 9-319164, wherein Vc[V] is a contrast voltage, Vo[V] is an initial electric potential, and S[μm] is a laser beam diameter.
Thus, even if the thickness of a charge transfer layer is comparable with the conventional one, the deterioration of a latent image is avoided and the resolution of the latent image is improved so that an image with a high density and a high fineness could be reproduced.
Also, the prior art disclosed in Japanese Laid-Open Patent Application No. 11-95462 is characterized in that a charge transfer layer of a photoconductor contains at least one kind of reaction product of a compound represented byR1m-M-(OR2)n, (M=Si, Al, Ti, Zr)
Thus, in sequential image formation with repeated charging and light-exposure, film chipping caused by wear and flaw of the layer is reduced and the layer has a high durability so that a photosensitive layer could be thinned. As a result, an electrophotographic photoconductor is provided on which a high quality image output with good tone and reproducibility could be obtained.
Next, an image formation apparatus using an electrophotographic process will be schematically illustrated.
FIG. 1 is a schematic diagram of a conventional image formation apparatus. A photoconductor drum 1 is formed by applying a photoconductor on the surface of a conductor and rotates in the direction designated by the arrow shown in FIG. 1. Image formation is performed by the following procedure in the image formation apparatus.                1. Charging means 2 electrifies the surface of the photoconductor at a desired electric potential.        2. Light-exposure means 3 exposes the photoconductor to light and forms an electrostatic latent image corresponding to a desired image on the photoconductor.        3. Developing means 4 develops the electrostatic latent image formed by the light-exposure means by toners and forms a toner image on the photoconductor.        4. Transcribing means 5 transcribes the toner image on the photoconductor to a recording sheet 6 such as a paper carried by a carrying means not shown in the figure.        5. Cleaning means 7 cleans toners that are not transcribed on the recording sheet by the transcribing means and remain on the photoconductor.        6. The recording sheet on-which the toner image is transcribed by the transcribing means 5 is carried into fixing means 8. In the fixing means 8, the toners are heated and fixed on the recording sheet.        
The photoconductor drum rotates in the direction designated by the arrow in FIG. 1 and desired images are formed on the recording sheets by repeating the aforementioned processes 1 through 6.
Conventionally, as a charging device in the electrophotorgaphic process, a corona charging device has been used, in which a photoconductor is charged by utilizing corona discharge. FIG. 2 is a schematic diagram of one example of the corona charging device. The material of the wire is tungsten and the diameter of the wire is 60 μm. The wire is extended and set at the position (the center of a charging case) as shown in FIG. 2 along the directions of the rotational axis of the photoconductor drum, on which wire a high voltage (approximately −7 kV) is applied. The wire is covered by the charging case The material of the case is a stainless steel that is not easily oxidized. Also, a grid is extended and set between the wire and the photoconductor, on which grid a voltage of approximately −0.6 kV is applied. The grid is provided by cutting a stainless steel plate (the thickness of the plate being 0.1 mm) into a mesh-shape.
In the corona charging device in FIG. 2, the charging of the photoconductor is performed as follows. In the neighborhood of the extended and set wire, a strong electric field is formed and dielectric breakdown of air occurs, to generate ions. A part of the ions are moved due to the electric field between the wire and the photoconductor, and the surface of the photoconductor is charged. Since the charging of the photoconductor is continued until the electric potential of the surface of the photoconductor becomes approximately equal to the electric potential applied on the grid, the electric potential of the surface of the photoconductor can be controlled by the electric potential applied on the grid.
There is also a corona charging device in which a sawtooth-shaped electrode is used as a discharge electrode, other than the corona charging device using a wire (Japanese Laid-Open Patent Application Nos. 8-20210 and 6-301286).
FIG. 3 is a schematic diagram of one example of the corona charging device using the sawtooth-shaped electrode. The sawtooth-shaped electrode has a shape as shown in FIG. 4, which electrode is made from a stainless steel plate with the thickness of 0.1 mm, wherein the pitch of the sawteeth is 3 mm. The sawtooth-shaped electrode is fixed on a supporting member as shown in FIG. 3, on which a high voltage (−5 kV) is applied by a power supply. Also, in the corona charging device using a sawtooth-shaped electrode, the electrode is covered by a charging case made from stainless steel and a grid is provided between the sawtooth-shaped electrode and the photoconductor, similar to the corona charging device using a wire. Also, charging of the photoconductor by the corona charging device using a sawtooth-shaped electrode is the same as the case of the the corona charging device-using a wire, and corona discharge occurs near the vertexes of the sawtooth-shaped electrode. In addition to the above those corona charging devices, a corona charging device in which a discharge electrode is a needle-shaped (pin-shaped) electrode has been devised.
The corona charging device using the sawtooth-shaped electrode has the advantages of more compact size and lower ozone generation compared to the case of the one using a wire. Since corona discharge by the sawtooth-shaped electrode creates an electric field stronger than electric field created by the wire (the flux of ions directed toward the grid or the photoconductor in the case of using the sawtooth-shaped electrode is lager than in the case of using the wire), the width of the charging device (or the width of an opening of the charging case at the side of the photoconductor) can be reduced. This is important for minituraization of the entire image formation apparatus. Also, since the corona discharge creates the stronger electric field and the flux of ions is larger, charging efficiency of the photoconductor is increased and the electric current flow through the corona charging device can be decreased. Consequently, the generation quantity of ozone is also reduced.
As a charging device for the image formation apparatus, there is a so-called contact charging device in addition to the above those corona charging device. The contact charging device can attenuate the problems of the corona charging device, that is,                1. much generated ozone        2. high applied voltage (5 through 7-kV).Accordingly, the contact charging device has been widely employed as a charging device for a low speed or middle speed electrophotographic process image formation apparatus.        
The contact charging device performs charging of the photoconductor by contacting a charging member with the photoconductor being a charged body (referred to as simply a photoconductor, below) and applying a voltage to the charging member. FIG. 5 is a sectional diagram of one example of the conventional contact charging device. A charging member 2 is roller-shaped with a diameter of 5 through 20 mm and a length of approximately 300 mm, on which an elastic layer 2a is formed on a conductor 2b. A photoconductor drum 1 has a diameter of 30 through 80 mm and a length of approximately 300 mm, on which a photoconductor 1a is formed on a conductor 1b. The charging member contacts the rotating photoconductor drum, and rotates following the rotation of the photoconductor. The elastic layer of the charging member is made from a material with the resistivity of 107 through 109 Ωcm. Then, a surface protecting layer with the thickness of approximately 10 through 20 μm may be formed on the surface of the charging member (the surface of the elastic layer). A voltage is applied on the charging member by a power supply 3 to perform charging of the photoconductor. The applied voltage is a direct current voltage of −1.5 through −2.0 kV. Due to such configuration, the photoconductor can be uniformely charged at −500 through −800 V by the contact charging device.
In the light-exposure means in the image formation apparatus using the electrophotographic process, light modulation in a so-called LD (laser diode) is performed corresponding to an output image. Laser light emitted from the LD is imaged onto the photoconductor through a so-called collimator lens, an aperture, a cylindrical lens, a polygon mirror, and an f-θ lens. The polygon mirror is a rotatable polyhedral mirror and laser light scans the photoconductor due to rotation of the polygon mirror. Accordingly, the photoconductor is exposed to laser light so that a latent image corresponding to a desired image can be formed on the photoconductor.
For the photoconductor of the image formation apparatus using an electrophotographic process, a so-called organic photoconductor has become popular. In the organic photoconductor, a lamination layer-type is popular, in which a so-called generating layer and a charge transfer layer are laminated on a conductive substrate so as to give a durability to the charge transfer layer. Furthermore, a protecting layer may be laminated on lamination layer-type organic photoconductors recently.
Moreover, since a demand for color printers have been advancing in recent years, it has become important to make the image quality higher.
In the image formation apparatus using an electrophotographic process, it is known that reducing the thickness of the photoconductor film is needed in order that the electric field for development can reproduce an image with higher spatial frequency (“Fundamentals and Application of Electrophotographic Processes”, Corona Publishing Co., Ltd., pp. 150-151).
However, as shown in a conventional technique (Japanese Laid-Open Patent Application 11-95462), when the thickness of the photoconductor film is reduced, the problem is that the durability of the film against wear and flaws due to cleaning is reduced and deterioration of the photoconductor film is accelerated by repetition of the charging process and light-exposure process. In the conventional lamination layer-type organic photoconductor, polycarbonate is generally used as a binder layer in the charge transfer layer, wherein the thickness of the charge transfer (CT) layer is generally set at approximately 20 through 30 μm due to the above-mentioned problem Accordingly, a CT layer with a thickness of 20 through 30 μm is used in actuality so as to maintain the high durability of the photoconductor film preferentially but sacrifice image quality.
According to an experiment performed by the inventors of the present invention, when a photoconductor having a charge transfer layer with the thickness of approximately 20 through 30 μm was employed, it was obvious that an image having a high spatial frequency, such as a so-called isolated 1 dot or 1 dot line image, could not be reproduced. Accordingly, a so-called bit-mapped image, etc. cannot be output without complex image processing by the image formation apparatus that does not fully reproduce the isolated 1 dot or 1 dot line image.
When the resolution of the image is reduced to 600 dpi or 400 dpi, the isolated 1 dot or 1 dot line image can be reproduced, but a coarse image is obtained due to the larger isolated 1 dot or 1 dot line. Also, reduction in resolution of an image including an oblique line causes jaggies, consequently degrading image quality. Furthermore, the problem for character images is that a resolution of equal to or more than 1200 dpi is required so as to discriminate between various fonts of the characters, and there has been the problem of simultaneously satisfying such high resolution of an image and reproduction of the isolated 1 dot or 1 dot line image.
Also, according to an experiment performed by the inventors of the present invention, when a photoconductor having a charge transfer layer with a thickness of approximately 20 through 30 μm was employed and image data subjected to a halftone processing at a line frequency of equal to or more than 200 lpi were written, the problem was that the output image had low tone so that an acceptable image could not be obtained for an image that requires tone representation at the same level as that of a photograph image. (On the other hand, when a halftone processing at a line frequency of less than 200 lpi is applied, the problem is that tone is maintained to be better but the texture of dithers is visible and a fine-grained image cannot be obtained.)
Moreover, in the condition of a worse tone (in this case of applying halftone processing with 200 lpi), the problem was that so-called banding was quick to occur and only a very noisy image was obtained.